Here's the issue...
When you enter the park's property, YOU become THEIR responsibility... YOU are now a paying guest and the park is liable for your safety... Why else is it that when things go wrong, the park are the ones deemed at fault even if it was down to guest stupidity at the end of the day...
So actually, because the park allowed Fred to re-ride Alpina Blitz several times without getting off, they are technically liable for the fact that he was ill... If he suffered any damage from this situation, then he could potentially sue the park for negligence for guest safety...
Just remember, parks create these rules to ensure the safety of YOU, the paying customer/guest... Because if they don't ensure the safety of their guests, they will be in a lot more trouble than it's worth...
I've done a fair few bouts of ERT in my time, and the ones I enjoy the most have been the ones where I've had to run around and 'queue' again... My ERT on Twister this year involved no getting off, and as a result I actually felt ill after about 30 minutes of constant riding, which could have easily spoilt my day and others if I had kept going...
BUT because I was the park's responsibility and it was them who gave us the ERT, who do you think is liable if I was ill?